growing mystery at one of America's most popular horse tracks, del mar in southern California. Ten horses died during season. Big changes are coming at the track. Mara schiavocampo has the latest. And away they go. Reporter: This morning, major changes at California's famous del mar racetrack after a streak of dangerous and in some cases deadly falls. It was very sad. They put the tarps up and euthanized the horse. This horse, chattering gambler died of an apparent heart attack, the tenth to die since the racing season opened at del mar three weeks ago. Three from natural causes, the others put down following catastrophic injuries on the brand new turf. When you have ten horses dying, and can't pinpoint one thing or another, it is a mystery. Reporter: Track officials shut down the turf course until Saturday to soften it and consult experts. And the synthetic surface will be replaced for next year. It's always about safety. Reporter: But track officials and experts say it may not be the track at all. A lot of rush to get a horse here. In combination with the pressure and the high purses, some horses may be running that aren't ready. Reporter: Still this could be a public-relations nightmare for the track, a huge tourist destination and hosting the breeders' cup in 2015. We will continue to follow that story.

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